Specialty pigments have been developed for use in security applications, such as anti-counterfeiting devices printed on banknotes, packaging of high-value items, seals for containers, and even for direct application to commercial items. For example, the U.S. twenty-dollar Federal Reserve Note currently uses optically variable ink. The number “20” printed in the lower-right corner of the face of the note changes color as the viewing angle changes. This is an overt anti-counterfeiting device. The color-shifting effect is not reproducible by ordinary color photocopiers, and someone receiving a note can observe whether it has the color-shifting security feature to determine the note's authenticity.
Other high-value documents and objects use similar measures. For example, iridescent pigments or diffractive pigments are used in paints and inks that are applied directly to an article, such as a stock certificate, passport, original product packaging, or to seals that are applied to an article. Security features that are more difficult to counterfeit are desirable as counterfeiters continue to become more sophisticated.
One anti-counterfeiting approach uses microscopic symbols on multi-layer color-shifting pigment flakes. The symbols are formed on at least one of the layers of the multi-layer color-shifting pigment flakes by a local change of an optical property(s), such as reflectivity. The multi-layer color-shifting pigment flakes generally include a Fabry Perot-type structure having an absorbing layer separated from a reflective layer by a spacer layer. The reflective layer is typically a layer of metal, which renders the pigment flake essentially opaque. If a large portion of these types of pigment flakes are mixed with other pigment, the resultant color might be significantly different from the pigment, and if too few of these flakes are mixed with other pigment, they might be difficult to find.
Another technique uses epoxy-encapsulated shaped flakes of polyethylene terephthalate (“PET”). A reflective layer is deposited on a roll of PET, and then the PET is cut into pieces. The flakes are coated or encapsulated with epoxy to improve the durability of the reflective layer. These flakes are available in a variety of shapes, such as square, rectangle, hexagon, and “apostrophe,” and a selection of reflective metallic tints, such as silver, pewter, gold, and copper. However, the epoxy layer and the relatively thick PET substrate (which typically has a minimum thickness of about 13 microns (0.5 mils) for use in vacuum deposition processes) result in a relatively thick flake, typically greater than 14 microns. Unfortunately, such a thick flake is not desirable for use in covert applications where the thickness is substantially greater than the base pigment. Similarly, such thick flakes do not flow well in inks, and create lumps in paint. When paint includes a thick flake that creates a rough surface, a relatively thick clear topcoat is typically applied over the rough surface.
It is desirable to mark objects with covert anti-counterfeiting devices that overcome the limitations of the techniques discussed above.
One aspect of this invention relates to providing polygonal shaped flakes wherein a plurality of flakes has a same shape. For example it would be desired to have flakes that are regular polygon shaped flakes, such as square shaped, rectangular shaped or triangular shaped. A particularly important aspect would be to have flakes with approximately the same shape. For example, such that a plurality of flakes could be recognized as square-shaped even though actual edges may differ slightly from the breaking process under which they are formed. In a most preferred embodiment of the invention, a plurality of flakes are formed by breaking or grinding a substrate into regular polygon shaped flakes, where all sides are of a substantially equal length and all angles forming the regular polygon are substantially equal.
Another aspect of this invention relates to providing flakes which have taggents or covert symbols stamped or embossed or etched therein by mechanical means or formed by laser means, wherein the covert symbols can be seen with a microscope. In order to preserve the integrity of the symbols, a frame is provided around all of or part of covert symbols so that when the individual flakes are removed from the support structure they are deposited on, the majority of flakes break along the frame lines provided instead of breaking in a less controlled unpredictable manner wherein break lines would otherwise occur with greater frequency through and about the symbols. In some instances parallel frame lines may be provided so that the flakes break into a ribbon; in a preferred embodiment of this invention, flakes and more particularly one or more symbols within a flake will have a framed grooved border on four or fewer sides about the one or more symbols, so that the flakes break in uniform squares or rectangles along the frame lines. Of course triangular or hexagonal flakes may be provided as well in this manner, by pre-framing symbols on three sides, prior to removing the flakes from their backing. A conventional release layer is provided so that the flakes can easily be removed from their backing or support layer and so that upon removal, the flakes break up along the faming lines. Frames can be made in a similar manner in which the symbols are made; using a laser, etching or stamping of the film that is upon the substrate; in a preferred embodiment, the frames are provided in the same process along with the formation of the symbols.
When flakes break into random shapes rather than into a regular polygon of a selected shape, the indicia can be damaged by way of the flake breaking along or through the indicia. This is shown in FIG. 6 and FIG. 8a of EP 0978373 in the name of Muller et al.
Although it may be possible to fabricate flakes in great quantities by cutting a substrate into flakes by use of mechanical cutters, it is preferred to have a method for fabricating shaped optical effect or pigment flakes, which is less costly and labour intensive by breaking the flakes along predetermined formed lines. The substrate prior to being crushed or broken has delineations therein which are visible and which facilitate the breakage of the flakes along predetermined lines.
It is a further object of this invention, to provide shaped flakes which may have symbols thereon, and wherein the symbols have or had, frames or borders embossed, or etched into the flake or formed with a laser beam for protecting the symbols during the process of separating flakes from their temporary support backing.
In one aspect, this invention relates to a method of producing flakes having covert symbols therein, wherein convert symbols within the flakes are substantially preserved in the process of removing the flakes from their temporary support backing.